Separation of hydrocarbons using zeolitic molecular sieves



p 1960 F. T. EGGERTSEN ETAL 2,952,630

SEPARATION OF HYDROCARBONS USING ZEOLITIC MOLECULAR SIEVES Filed Nov.19, 1956 owmu INVENTORI FRANK T. EGGERTSEN JOHN W. GIBSON THEIR ATTORNEYUnited States Patent 2,952,630 SEPARATION OF HYDRGCARBONS USING ZEO-LITIC MOLECULAR SIEVES Frank T. Eggertsen, Orinda, and John W. Gibson,Oakland, Califi, assignors to Shell Oil Company, New York, N.Y., acorporation of Delaware Filed Nov. 19, 1956, Ser. No. 622,894 6 Claims.(Cl. 208310) This invention relates to a process for the separation ofnormal hydrocarbons from branched and/or cyclic hydrocarbons. it relatesmore particularly to an improved vapor-solid contacting process for thefractionation of hydrocarbon mixtures containing a plurality of bothnormal, i.e. linear, hydrocarbons and non-normal hydrocarbons.

Various methods have been proposed heretofore for the selective removalof more polar hydrocarbons from mixtures containing them and less polarhydrocarbons, by' the use of solid adsorbents which exhibit selectiveadsorptivity for the more polar substances. Thus, aromatics areselectively adsorbed by silica gel, activated carbon, alumina, and thelike and are separable thereby from non-aromatics, both cyclic andacyclic. The socalled Arosorb process thus uses silca gel to recoveraromatic hydrocarbons, such as benzene, toluene and the xylenes, frompetroleum naphtha streams, such as gasoline boiling range fractionsproduced by catalytic reforming, as by platforming. Except in specialcases, however, equally satisfactory processes for the separation ofstraight-chain hydrocarbons from non-straight-chain hydrocarbons havenot been made available.

Normal parafiins are frequently undesired components in hydrocarbonfuels and lubricants: they depress the octane number of gasoline, raisethe pour point of lubricating oil, and raise the freezing point ofdiesel and jet fuels.

It has been proposed to utilize certain natural and synthetic zeoliteshaving rigid three-dimensional anionic networks and havingintracrystalline interstitial channels sufliciently large to accommodatestraight-chain hydrocarbons but sufficiently small to excludebranched-chain and/or cyclic hydrocarbons, to separate straight-chainhydrocarbons from other hydrocarbons (Barrer, US. 2,306,610; Black, U.S.2,522,426). Although normal hydrocarbons are selectively sorbed by suchsolid substances, no commercial process has been developed to utilizethis separation effect. Although the rate of sorption of the normalhydrocarbons is rapid, the regeneration of the sorbent for reusegenerally is relatively slow and time-consuming so that the large-scaleapplication of the separation effect is economically infeasible.

It is a principal object of the present invention to provide an improvedprocess for the separation of normal hydrocarbons from hydrocarbonmixtures containing them. It is a more specific object to provide animproved process for the separation of normal hydrocarbons from ahydrocarbon naphtha fraction which contains a plurality of normalhydrocarbons in admixture with non-normal hydrocarbons. These objectswill be better understood and others will appear to those skilled in theart from the more detailed description of the invention which will bemade with reference in part to the accompanying drawing, wherein:

Figure 1 is a schematic flow sheet of one mode of practicing thisinvention; and I Figure 2 is a schematic flow sheet of another mode ofpracticing this invention.

Zeolites having rigid three-dimensional anionic net- 2,952,630 PatentedSept. 13, 1960 works and having intracrystalline interstitial channelswhose narrowest cross section has essentially a uniform diameter, e.g.about 4 or 5 Angstrom units, are well known to the art. They arecommonly designated molecular sieves. The intracrystalline channels aregenerally designated pores. Such zeolites are described, for example, ina paper, entitled Zeolites as Absorbents and Molecular Sieves, by R. M.Barter in Annual Reports on the Progress of Chemistry for 1944, vol. 61,pp. 31-46, London (1945). Another molecular sieve is described by Blackin U.S. 2,442,191. More recently certain synthetic molecular sieves havebecome commercially available from Linde Air Products Company. One suchmolecular sieve is designated MS-4A. It is a zeolite of averagecomposition 0.96:0.04 Na O, 1.00 A1 0 1.92:0.09 SiO plus an amount ofwater depending on the degree of dehydration; the crystals are cubic,with unit cells measuring, on an edge, approximately 12.26 Angstromunits, and are characterized by an essentially uniform pore diameter ofabout 4 Angstrom units. Another available sieve is designated MS-SA.This is made from MS-4A by replacement of approximately of the sodiumions with calcium ions by ion exchange. These are also cubic crystals,having the same unit cell dimensions as MS-4A, and are characterized byan essentially uniform pore diameter of about 5 Angstrom units. Thecrystallites of Lindes sieve materials generally have diameters of from500 to 5000 Angstrom units.

Zeolites become active for selective sorption by a treatment designed todrive ofi the water originally present in the interstitial spaces. Thespaces vacated remain and become available for the sorption of compoundsof appropriate maximum critical molecular cross section. The zeolitesmay be subjected to temperatures of 600 C. and, in some cases up to 800C. or more without destruction of their crystalline structure. In somecases, repeated contact with steam can be tolerated Withoutsubstantially affecting their structure.

Activated zeolites are generally soft, friable materials. Although theymay be used as sorbents in pure form, if carefully handled, they can bemade up in the form of particles held in shape bythe addition of ten totwenty percent of an inert binder material such as clay. They may alsobe used admixed with other solids which are stable at the conditions tobe used in the process and which are, preferably, non-adsorbents, orrelatively nonselective adsorbents so as not to interfere in the desiredseparations.

The basis for the separation of the normal-hydrocarbons from hydrocarbonmixtures containing them and non-straight-chain hydrocarbons which iseffected by these natural or synthetic zeolites appears to be that thenormal hydrocarbons are capable of passing into and through theinterstitial openings (pores) of the sorbent, Whereas thenon-straight-chain hydrocarbons have too large a maximum cross-sectionaldiameter to do so. This is supported by the results and the followingapproximate largest cross-sectional dimensions perpendicular to thelongitudinal axes, sometimes designated critical cross sections, asdetermined by the use of Fischer-Hirschfelder scale models, reputed togive fair estimates of molecular size, of some representativehydrocarbons: nparafiins, 4.9 Angstroms; monomethylparaifins, 6.3Angstroms; gem-dimethylpar-aflins, 6.7 Angstroms; ethylparafiins, 7.2Angstroms; cyclohexane, 6.6 Angstroms; and benzene, 6.9 Angstroms. Thesedifferences in critical cross-section permit the separation of normalhydrocarbons from branched and cyclic hydrocarbons by sorption inmolecular sieves whose pore diameters are sufficiently large to admitthe normal hydrocarbons but not large enough to admit the other types.Thus, chabazite,

- 'It should be understood that pore diameters herein referred to, whichare determined by physical measurements such as X-ray methods or by thesorptive characteristics of the zeolites,'rnay not be precisely accuratenumbers. Also, it has been found that normal hydrocarbon molecules mayenter pores whose smallest diameters are believed to be at leastslightly smaller than the apparent maximum critical cross-sectiondiameter of the molecule. For purposes of this invention, reference tozeolites having substantially uniform intracrystalline interstitialchannels (or pores) of from about 5 to about 6 Angstrom units diameterincludes those above-mentioned zeolites and otherswhich'h'ave thecharacteristic of selectively sorbing normal hydrocarbons of four or.more carbon atoms per molecule and notsorbing non-normals, by virtue oftheir crystal structure. These sorbents may also be; referred to aszeolitic molecular sieves of from about 5 to about 6 Angstrom unitmaximum pore diameter;

The necessity for substantial uniformity of the maximum pore diametersis demonstrated by the fact that when a silica' gel having the sameaverage pore diameter as the molecular sieve, but a distribution ofsizes including substantial proportions both of smaller and largerpores, is used in the place of the molecular sicve,;a suitableseparation is not obtained. a

The pore volume in molecular sieves is quite substantial. It may be asmuch as about 50% of the total volume of each crystal of sorbent. Theweight of normal hydrocarbons which can be sorbed is a function of theavailable pore volume, the temperature, pressure and other j conditionsincluding the concentration of the normal hydrocarbon in the mixture andthe rate at which the mixture is passed in contact with the sorbent. Itmay be as high as 14% by weight of the sorbent. When operating in vaporphase at temperatures above 250 C. and at atmospheric pressure, thedynamic capacity of a molecular sieve such as Linde MS-SA for normal.hydrocarbons may be in the range from 3 to 6% by weight, or higher,based on the weight of sorbent. The capacity is greater for hydrocarbonsof higher molecular weight; it increases at higher pressures butdecreases. at higher temperatures and at increasing flow velocitiesthrough the bed of sorbent.

Heretofore, it has been considered very important to utilize molecularsieve capacity to its fullest extent when separating .no-rmalhydrocarbons from admixture with other hydrocarbon types. This however,required the use of long desorption periods or of severe desorptionconditions such as increased temperature and rapid gas flows.Surprisingly, it has now been found that a greatly superior method forthe separation of normal hydrocarbons from admixture with other typesconsists in utilizing only from one tenth to one half of the capacity ofmolecular sieve sorbent for normal hydrocarbons. V V

In accordance with this invention, a mixture of hydrocarbonsrfrom whichnormal hydrocarbons of at least four carbon atoms per molecule are to beseparated is passed in contact with a mass of molecular sieve sorbenthaving a pore diameter of from about 5 to about 6 Angstromsin'relatively brief pulses, followed by relatively brief periodsofsweeping the mass of sorbent with an inert sweeping gas; the pulses offeed and the conditions of desorption-are so'correlatedthat'no more thanone tenth to one half of sorbed normal hydrocarbons is removed in thesweeping periodand the amount of normal hydrocarbons in each pulse offeed is approximately equal to the actual capacity of the mass ofsorbent for normal hydrocarbons at the beginning of the feed pulse, i.e.about one tenth to one half of the total capacity of the sorbent.

Expressed another way, this is a continuous process for the separationof normal hydrocarbons from admixture with non normals in which portionsof'feed are intermittently passed in contact with a mass of suitablemolecular sieve sorbent which, once the process is in continuousoperation, contains from one half to nine tenths of its capacity ofnormal hydrocarbons at the beginning of each feed pulse and issubstantially full of normals at the end of the feed pulse and thebeginning of the sweeping gas pulse. 7

When a pulse of feed hydrocarbon mixture is charged to the sorbent bed,the non-normal hydrocarbons in the mixture are not delayed in theirpassage through the sorbent mass and consequently they emerge from themass very rapidly, depending only on the rate at which the mixture ischarged. The concentration of the nonnormal hydrocarbons in the efiluentvapor mixture is higher than in the feed vapor mixture passed to thesorbent since the normal hydrocarbons are being retained in the sorbentmass. The non-normals are, therefore, present in the effluent vapors'ina relatively high concentration and are readily condensed therefrom byconventional condensers operating at suitable temperatures e.g., between15 and 40 C. When the predetermined amount of feed has been charged inany one pulse and further addition of feed is discontinued, theconcentration of non-normals in the effiuent decreases very rapidly toessentially zero, as soon as all the non-normals have been swept throughthe contact mass. During the following period, when only sweep gas ischarged to the contact mass, norm-a1 hydrocarbons are desorbed irom thecontact mass and appear in the efliuent vapors in relatively lowconcentration compared to the concentration in which the non-normalsappeared.

In one modification of this invention, the effluent vapors are passedthrough a condenser operating at a relatively higher temperature duringthe period when anon-normals are present in the efiluent, whichessentially coincides with the feed pulse period, and the effluentvapors are then switched to a different condenser operating at a lowertemperature e.g., between 0 and C. 'to recover the normal hydrocarbonspresent in the efliuent vapors during that period.

In another mode of practicing this invention, advantage is taken of thediiference in concentration of the nonnormals and the normals in theeffluent vapors by passing the total product mixture continuouslythrough a first condenser which is so adjusted in temperature. thatsubstantially all of the hydrocarbons in the efiiuent are condensed whenthe non-normals are present but'the normals are not condensed when theyare present alone in low concentration during the sweeping portion ofthe cycle, and the gas mixture leaving the first condenser is thenpassed through a second condenser operating at a substantially lowertemperature to condense all me remaining hydrocarbons from the sweepgas. In this mode, the receiver for the first condenser collectsessentially all the non-normal hydrocarbons and the receiver for thesecond condenser collects essentially only the normal hydrocarbonspassing through when there are no non-normals present, as well as thesmall amount of uncondensed nonnormals-which pass through the firstcondenser in the feed pulse of the cycle.

This invention takes advantage of .the fact that the rate of sorption ofnormal hydrocarbons on passage of a hydrocarbon mixture containing theminto contact with a molecular sieve is rapid and that the normalhydrocarbons are held bythe sieve'quite tenaciously so that o it is notpossible to completely desorb normal hydrocarbons from the sieve whenapplying only heat and vacuum except by using very high temperatures orextremely long desorption periods, it does appear that the normalhydrocarbons are not entirely held in place on the sieve mass but thatthey gradually diffuse through the sieve in the direction of lowernormal paraffin concentration or in the direction of flow of the vapormixture. It is, therefore, possible to desorb the normal hydrocarbonsfrom a molecular sieve mass by passing a so-called sweep gas through thecontact mass. The normal hydrocarbons are then contained in the totalvapor mixture leaving the bed. At a given temperature, the amount oftime and the amount of sweeping gas required for recovery of a givenamount of a sorbed normal hydrocarbon are interchangeable variables.Thus, a very small amount of sweeping gas passed slowly through a bed ofnormal-rich sorbent permits removal of the same amount of normalhydrocarbons as a large amount .of sweeping gas passed through a bed ina much shorter period of time, conditions otherwise being equal.

It is characteristic of the process of this invention that, since thenormal hydrocarbons are not completely removed from the sorbent bedbefore additional feed is charged, the efliuent from the sorbent bedcontinuously contains a small concentration of normal hydrocarbons. Itis, therefore, not in the nature of the process of this invention tomake a complete separation of all normal hydrocarbons from a mixturecontaining them. It is, however, possible to reduce the concentration ofnormal hydrocarbons in the non-normal portion of the eflluent to a lowvalue, e.g., below and preferably in the range of from 2 to 5%, bysuitably adjusting the length of time of the feed pulse, the flow rateof the feed and sweeping gas during the sweeping period and thetemperature and pressure maintained in the sorbent bed as Well as thoseof the product condensers mentioned above.

In the process of this invention the temperature of the molecular sievesorbent mass is suitably maintained in the range from 250 to about 600C. It is better to maintain a temperature of at least 350 and no morethan 500 C.; temperatures between 350 and 450 C. are preferred.

Within the above range, the temperature is generally chosen sufiicientlyhigh that the ratio of the absolute temperature in the contacting zoneto the absolute boiling temperature of the highest boiling component inthe feed mixture at the operating pressure is at least 1.1. It ispreferably chosen to be below the temperature at which the least stablehydrocarbon in the feed undergoes appreciable thermal cracking at theprocess conditions. The thermal stability of the sorbent must not beexceeded.

It is an advantage of this invention that the separation process can becarried out isothermally, i.e., the temperature of the sorbent mass canbe maintained essen tially unchanged throughout each cycle. There may besome difference in the temperature during the adsorption and desorptionof normals, e.g., up to 100 C. For example, the feed may be relativelycool, thus cooling the sorbent during the charging step, or the heat ofsorption of normals from a warm feed may heat the sorbent during thecharging step. The temperature of the sorbent may be maintained byindirect heating or cooling means or by control of the feed andstripping gas temperatures.

The pressure is suitably in the range from atmospheric pressure to 1000lbs/sq. in. gauge, and preferably between 100 and 750 p.s.i.g. Theefiect of operating at the higher pressures in this range is to increasethe capacity of the bed for sorbing normal hydrocarbons, but at the sametime to increase the actual quantity of sweeping gas required (measuredat standard conditions) in the desorption step. Pressures above 1000psig. can, therefore, be employed if the amount of sweeping gas requireddoes not become uneconomically high.

In operating according to this invention the amount of 6 normalhydrocarbons sorbed in the mass of molecular sieve sorbent does not fallbelow of the capacity of the sorbent, once continuous operation has beenestablished. It is preferable to operate with a sieve mass containing atleast about two-thirds of its capacity of normal hydrocarbons at alltimes and the lowest normal hydrocarbon content of the sieve mass incontinuous operation may be as high as nine-tenths of its total dynamiccapacity. Total dynamic capacity here refers to the amount of normalhydrocarbon which is retained by the sieve when a hydrocarbon mixture ofthe composition to be charged is passed into contact with activatedmolecular sieve sorbent, either fresh or regenerated so as to becompletely free of normal hydrocarbons, at the conditions oftemperature, pressure and gas flow to be employed in the separationprocess.

The amount of feed charged to the sorbent mass in each pulse duringcontinuous operation is determined by the working capacity of thesorbent mass for normal hydrocarbons at the time the feed pulse isstarted. The length of the feed pulse may be varied by varying the rateat which the feed is added. Feed pulses as short as from one to a fewseconds have been found satisfactory in small scale experiments, sinceselective sorption of normal hydrocarbons is practically instantaneousat the conditions of this process. It is convenient to control the feedpulse such that the feed is added at a LHSV of up to 20 v./v./hr.(measured during the feed pulse). Any lower rate is operative, e.g.,down to l v./v./hr. or less, but rates from 5 to 20 v./v./ hr. arepreferred. Much higher rates may be used; limits are set mainly byapparatus limitations. The length of the desorption period is determinedaccording to the extent of desorption desired; it is affected by factorswhich have already been discussed, e.g., the temperature, sweep gas rateand type of hydrocarbon to be desorbed. It is convenient to fix thedesorption period at a value such that the feed rate for the overallprocess is at a LHSV between 1 and 4 v./v./hr. or higher, preferablybetween 2 and 4 v./v./l1r. The time during which normal hydrocarbon isdesorbed is generally at least as long as the time during which feed ischarged and may be up to ten times as long or longer. The time requiredin the desorption step can be shortened by increasing the flow rate ofsweep gas through the sorbent mass during the desorption step. 7

In some cases it may be desirable to make no attempt to remove thelowest molecular weight normal hydrocarbons from a feed stock. In thatcase the process is modified in such a manner that-the sorbent capacityis considered and used only for the higher normal hydrocarbons which areto be separated, while the lower normal hydrocarbons are permitted torun through the bed at all times and be recovered with the non-normalhydrocarbons.

Figure 1 illustrates a single mode of practicing the present invention.The molecular sieve sorbent, such as Chabazite or Linde molecular sievetype 5A which has been activated by driving off most of the water ofhydration, is placed in a fixed bed A. Efiluent from the bed passes toproduct recovery zone B or C. Zones B and C may each contain a condenserand product receiver or other suitable recovery means, such as ahydrocarbon absorber or adsorber. In starting up the process, sweep gasis passed through line 11 controlled by valve 12. The gas passes intosorbent bed A which is maintained at a desired predetermined temperatureby the gas or by other means, not shown. Gas leaves bed A through line15 and passes through line 16 controlled by valve 17 into productrecovery zone B and out through line 18. Valve 20 in line 19, leading tozone C, is kept closed at this time. Hydrocarbon feed mixture,preferably vaporized and preheated, is then added to line 11 throughline 13 controlled by valve 14. Flow of feed mixture through bed Aresults in holdup of normal hydrocarbons from the feed mixture in themolecular concentration than were the non-normals.

sieve .bed A while the non-normal hydrocarbons pass through and areremoved from the inert gas and retained in zone B... The sweep gascontinues to leave zone B through line 18. Flow .offeed is continueduntil the molecular sieve mass is substantially filled with normalhydrocarbons. This may be determined by calculation from knowledge ofthe capacity of the molecular sieve at the conditions of the process andof the content of ,normalfhydrocarbons in. the feed or it may be:determined by a continuous or intermittent analysis of the effluentpassing through line for normal hydrocarbon content. When the sieve massis substantially filledv with normals, the flow of feed is discontinuedby closing-valve 14 and-shortly thereafter, when the remainingnon-normal hydrocarbons have been flushed out of the bed, valve 17leading to zone B is closed and valve 20 in line 19, leading to zone C,is opened. Conditions in zone C are. adapted to recover the normalparaflins now being swept out of the mass of molecular sieve and presentin the effluent therefrom in much lower At the same time, if desired,desorption of normal hydrocarbons may be accelerated by substantiallyincreasing the rate of sweep gas flow by manipulatingvalve '12 in linell, or by raisingthe temperature of bed A; the latter is generally an.uneconomic procedure. Normal hydrocarbons are collected in zone Cduring-this period while essentially hydrocarbon-free sweep gas passesout through line 21. After from one-tenth to one-half of the normalhydrocarbon capacity of the molecular bed has been de-' sorbed, a newpulse of feed is charged by opening valve 14 in line 13; if the sweepgas rate was increased during the desorption step it is again lowered tothe desired value for the charging pulse by closing down on Valve 12.Valve 17 is opened, valve 20 is closed, and nonnormal hydrocarbons areagain recovered in zone B. The feed pulse is continued until'asufiicient amount of normal hydrocarbons has been charged tosubstantially fill the capacity of the molecular sieve mass for normals,as determined by calculation from knowledge of the capacity of'thesieve, the feed rate and the concentration of normals in the feed, or byanalysis of the efiiuent in line 15.

As has been previously pointed out, some normal hydrocarbons appearcontinuously in line- 15 during continuous operation of the process, butthe concentration is relatively low as long as the bed has capacity forsorbing additional normal hydrocarbons. If the flow of feed is continuedafter the bed capacity is filled up at the conditions of operation, thenormals will pass through essentially at the rate at which they are fed.When the saturation point is passed, the concentration ofnormals in thevapors in line 15' will show a sudden increase from a relatively lowcontinuous rate and will quickly approach the concentration in which thenormals are present in the feed.

'In the continuous operation, the feed pulse with recoveryof theefiluent in receiver C and the sweeping step with recovery of theeffluent in receiver D are cyclically repeated. The'cyclic operation maycontinue for many days without requiring further treatment of thesorbent bed, provided the conditions of temperature, pressure, rate andso forth are controlled within the ranges given in this specificationand the feed contains no impurities which tend 'to poison the sorbentbed. The product receivers in zones B and C are periodically emptied ormay be continuously emptied through valved lines 22 and 23respectively.

Numerous modifications of the system shown in Figure 1 will occur tothose skilled in this art. For example, instead of providing separateproduct recovery means in zones B and C a'single means may be placed inline 15; zones B and C, in that case, merely provide productaccumulators. The recovery means in line 15 may consist ofja singlecondenser operating at a temperature suf- 8 ficiently low to permitadequate condensation. of normal hydrocarbons during the desorptionstep, or of a single condenser operating at diiferent temperatures forthe recovery of non-normals and of normals,.or it may comprise twocondensers operating at difierent temperatures, connected in such mannerthat the non-normals are recovered at. a relatively higher temperaturethan the normals.

A preferred modification is illustrated schematically in Figure 2. Theapparatus shown in Figure 2 comprises a feed tank E; a sorbentbed Flocated in a heatable vesselG; a first product condenser H; a productanalyzer I adapted to determine the concentration of normal hydrocarbonsin a vapor mixture; A means I for recovering. the remaining hydrocarbonsfrom a vapor stream, which may be a condenser or an absorber;productaccumulators K and L, and product receivers M and N. Since theprocess diagram is schematic in nature, much necessary associatedequipment which can be readily supplied by those skilled in. the art isnot shown to avoid unnecessary complexity.

In starting up the process, a flow of inertgas is commenced through line211'- controlled by'valve 212. This gas passes through bed P, which isbrought to a desired temperature by passing heating medium through thejacket, the heating medium entering through line 214 and passing outthrough line 215. Other means of heating the bed F may be employed,e.g., liquid coils imbedded in the sorbent bed or electrical or othermeans. The gas may be used as a direct heating medium. The gas passesout of the bed through line 215, through condenser H, line 218,accumulator K, line 219, condenser or'absorber J, line 220, accumulatorL and finally through line 221. It may be taken out of thesystem throughline 222, controlled by valve 224, or it may be returned to line 211,for further use, by means of line 225 containing compressor 0. Once theflow of gas is established at a desired rate, feed hydrocarbon mixtureis passed from tank E through line 226 containing vaporizer 228 and intoline 211. As the resulting mixture passes through the bed, normalhydrocarbons are retained in bed F and nonnormal hydrocarbons are sweptout through line 216, condensed in condenser H and recovered inaccumulator K. The condenser temperature is maintained by controllingthe rate of flow of cooling medium which enters the condenser throughline 229 and leaves it through line 231. The flow controlling means isvalve 230, which may be manually adjusted or may be regulated fromanalyzer I once continuous operation is established. Analyzer I may be,for example, a differential refractrometer which continually determinesthe difierence in refractive index between amixture having thecomposition corresponding to original feed minus the amount of normalhydrocarbons to be removed therefrom and the actual liquid condensatewhich has been collected in at least one complete cycle of operation invessel K; the condensate is sampled through'line 232. The differentialreading is used to control valve 230, e.g., by controlling theair-pressure supplied to the valve to open it further and thus lower thetemperature of the condenser. for additional hydrocarbon recovery or toopen it and raise the condenser temperature, thus keeping theconcentration of normals in the recovered liquid in a predeterminedrange.

The flow of feed is continued until the capacity of sorbent bed F fornormal hydrocarbons is substantially used up. This may be calculatedfrom a knowledge of bed capacity at the conditions employed, theconcentration of normal hydrocarbons inthe feed and feed rate. It mayalso be determined by analyzing the vapor mixture in line 218, e.g., bypassing a continuous sample of the mixture to an analyzer similar toanalyzer I.

When the sorbent bed F has been filled essentially to capacity withnormal hydrocarbon, flow of feed mixture is discontinued by closing'valve 227. in. line 226 and the 9 flow of gas is continued. The resultis that the condensation of non-normal paraflins is quickly completed;the efliuent vapors from bed F thereafter pass through condenser H andaccumulator K without change and the normal hydrocarbon content in thevapors is substantially completely recovered by further condensation ata lower temperature or by absorption in a suitable absorbent liquid inrecovery vessel J. The resulting normal hydrocarbon liquid is retainedin accumulator L. The sweep gas substantially free of hydrocarbonspasses through line 221. Preferably, the sweep gas is returned forfurther use through line 225. After a predetermined relatively shortperiod of desorption in which the amount of normal hydrocarbon removedfrom bed F is equal to one-tenth to one-half of the bed capacity, theflow of feed to bed F is re-established by opening valve 227. Thisautomatically results in non-normal hydrocarbons appearing in theeffluent line 216 in substantial concentration and again being condensedin condenser H and recovered in accumulator K.

It is advantageous to utilize at least two separation columns inparallel, manifolded so that difierent steps of the process can becarried out in different units simultaneously and processing streams ofhydrocarbons and of sweep gas move substantially continuously throughthe lines to and from the manifolds.

The sieve material may be provided in a suitably sized vessel, generallyan upright cylindrical vessel with The material a length from 2 to 10times the diameter. is suitably supported by a screen grid in the bottomand, if desired, a plurality of suitable screen grids are prov'ided atspaced intervals throughout the column to sup- 7 port the sievematerial.

By supporting the sieve material on a plurality of 7 screen grids, asmall zone of free space can be left between the top of sieve materialdisposed on one grid and the underside of the next grid thereabove. Inthat case, advantage can be taken of reduced pressure drop resultingfrom passing the larger volume of eluting gas a through the sieve massunder conditions less likely to cause packing of the zeolite mass andtending to expand the volume of the mass.

The bed of zeolite material may be disposed horizontally instead ofupright and it may be provided as a fixed, stationary mass, or it may beadapted to be moved as a mass, as in an elevating system or as annularsegmental packing of a rotatable vertical or horizontal contactorproviding solid particulate contacting material in the annular spacebetween concentric rotatable cylindrical screens or perforatepartitions, suitably provided with means for delivering and removingfluid streams to and from outer and inner surfaces of the mass andperiodically to change the nature of the fluid delivered to and removedfrom any particular segment and to reverse the flow of fluidtherethrough.

In the normal hydrocarbon removal step of this invention helium,hydrogen, nitrogen and methane can be sucessfully used as sweep gases.Other inert gases, i.e., gases which do not react with either thesorbent, the vessel or the. reactants, are also suitable. For example,

argon, flue gas (preferably scrubbed to remove reactive impurities),propane and other gases and vapors can be used as sweep gas.

It is not essential to use a sweep gas while the hydrocarbon feed isbeing charged. The feed may be added to the heated sorbent as a liquid,to be quickly vaporized by contact withsorbent, or it may be added as avapor. 7

If desired, sweep gas flow may be maintained through the sorbent mass atall times and the feed added as liquid or as vapor to the sweep gas. Itwill generally be preferred to maintain a relatively low rate of inertgas flow or no gas flow at all during the time when feed is added,

and a substantial gas flow during the time when sorbed normalhydrocarbons are desorbed. The first sweep gas added after feed isdiscontinued serves to flush the 7 10 remaining non-sorbed hydrocarbonsout of the sorbent mass. The rate of gas flow during this flushing stepmay diifer from that used during the sweeping step.

The process of the present invention is useful in the separation ofnormal hydrocarbons from mixtures containing normals having a carbonnumber of at least 4 and generally at least 5. It is particularlysuitable for mixtures containing normals from n-pentane or n-hexanethrough n-decane or n-dodecane and corresponding olefins, but may beused with mixtures containing normals up to n-eicosane.

The invention will be further illustrated by means of the followingexamples, in which Example I illustrates runs carried out in accordancewith the mode described by means of Figure 2, and Example II a runcarried out in accordance with the mode described by means of Figure 1.

The feed used throughout these runs was a dehexanized C -C Platformatefraction. This fraction had a refractive index (R. I.) of 1.4538, andcontained about 41% wt. saturated compounds, 59% wt. aromatics and 0%wt. olefins. The content of normal parafiins was about 12% wt.

EXAMPLE I Sorbent bed'F consisted of ca. parts by weight of Lindemolecular sieve sorbent type 5A, in the form of irregularly shapedparticles of average 1-2 mm. diameter, containing about 80% wt. of theactual zeolite and 20% of a clay binder. The bed was placed in anexternally heated vertical cylindrical vessel of about 20:1 height todiameter ratio.

Throughout the run, the bed was maintained at a temperature of about 450C. A flow of dry nitrogen through the bed was maintained throughout therun at a rate of about volumes (at STP) per bulk volume of sorbent perhour. The nitrogen was passed in at the top of the sorbent bed.

Condenser H was maintained at about 18 C. by

' means of cooling water, and condenser J at about -l80 C. by means of arefrigerant.

The feed was added to the flowing nitrogen at the rate of 0.72 liquidvolume per bulk volume of sorbent per hour, in portions of 20 parts byweight, each, during a period of 6.7 minutes. After each feed pulse,flow of nitrogen without feed addition was continued for 20 minutes. Thenon-normal product was separately recovered from receiver K after eachfeed pulse; it was measured and analyzed for normal paraflins by meansof refractive index. The hydrocarbons not recovered in receiver K wererecovered in receiver L. This product was measured only at the end ofthe run, i.e. after several cycles had been completed, and its volumeand refractive index then determined. a

The results of the first run, in which the feed was charged in pulses of20 parts by weight, are set out in Table I.

Table 1 Product 1st Condenser (H) Feed- Analysis Cycle Weight Nor-(parts) Weight Percent mal Par- (parts) of Feed afiins,

(wt.) percent Product 2d Condenser (J) It became'quicklyapparent;merelyfrom an inspection of the refractive index of the product fromreceiver K, that normalparaflins were being removed only to anundesirably small extent. This was an indication that the feed pulseswere too large, exceeding the capacity of the bed for normal parafiins.

-Insanother run in which the same conditions were maintained exceptthat'the amount of feed charged in each pulse was only about 10.0 partsby weight, it was found that the capacity of the bed was not exceeded.The results of this run are set out in TableII. This run was continued for15 cycles, and was then voluntarily termi- 12 volumes per bulk volumeof sorbentper hour, in portions ofca. parts by weight, each. Feedaddition in each cycle lasted ca. seconds, flushing of non-normals ca.15 seconds and desorption of normals ca. seconds. Be cause of the rapidfeed rate, the sorbent bed gradually cooled from an inlet temperature ofca. 525 C. to one of ca. 300 C, at the end of 10 cycles. Flow wasdiscontinued for 5 minutes after each set of ten cycles'to permit thebed to be heated to its original temperature. The

10 average bed temperaturewas about 450 C. Recovery means B represents acondenser and reactor operating'at 18 C.,'followed by a set of coldtraps at --180 C. Almost 98% of the normal-lean product was recoverednated.

Table 11 Product 1st Condenser (H) Feed-- Analysis Cycle Weight (parts)Weight Percent (parts) of Feed Normal Satu- Aroma- Olefins, (wt.)Paraflins, rates,'pertics, perpercent percent cent wt. cent wt wt.

10.0 8. 3 83 3. 7 10.0 8. 7 r 87 3. 5 10.0 8. 7 87 3. 5 10.1 9.3 V 924.8 9. 9 8.8 89 5.0 10.0 8. 4 84 4.9 9.9 9.0 91 5.2 14. 7 13. 3 91 4. 99. 9 8.7 88 5. 7 10.0 8. 5 4.4 9. 9 8. 1 82 4. 6 l0. 5 8. 8 84 4. 8 10.08.6 86 3.7 10.0 s. e as 4.4 10. 4 8. 1 78 4. 1

Product 2d Condenser (3') EXAMPLE ,II

Sorbent bed A was of the same size and shape as bed F of Example I. Therun consisted of 5 0 cycles. In each cycle, dry nitrogen flow wasmaintained, but the rate was varied from 270 volumes (at STP) per bulkvolume in the condenser-receiver.

bed of silica gel.

50 The results of this run are set out in Table III.

Recovery means C represents a set of cold traps operating at -180 C.followed by a Non-normal product was-removed after each set of tencycles; it was measured and analyzed for normal parafiins content bymeans of refractive index. The product in cold traps C was measured andanalyzed at the end of the Table III Product, Condenser B Feed CycleCharged, 7 Normal (pts. Percent Paraflius Satu- Aroma- Olefins, wt.(pts) wt of Content, rates, pertics, perpercent 7 feed percent cent v.cent v. v.

110..... 78. 6 78. 6 11-20-..- a 100 81. 6 81. 6 3. 7 21-30--.. B 10082. 0 82.0 3. 7 31-40 a 100 81. 7 81. 7 3. 7 4150 H 100 82.3 82. 3 3. 7

Con-

denser Tot 406.2 From Cold Trap-.- 9. 3

Total. V 493. 6 415. 5 84. 6 3. 7

. Product Cold Traps C l-50. 493.6 69.8 14.2 s 50 72 r '28 0 We claim asour invention:

1. A process for the separation of normal hydrocarbons from a mixture ofa plurality of normal hydrocarbons of at least four carbon atoms permolecule with non-normal hydrocarbons of similar boiling range whichcomprises: (1) passing a vapor stream of the hydrocarbon mixture into aparticulate fixed mass of a solid zeolitic material having a rigidthree-dimensional anionic network and having substantially uniformintracrystalline interstitial channels of from about to about 6 Angstromunits diameter while maintaining a contacting temperature of from 250 C.to 600 C. to selectively retard the passage of at least the highermolecular weight normal hydrocarbons through the mass while theremaining hydrocarbons pass through the mass and are recovered as anon-normalhydrocarbon-enriched product; (2) discontinuing passage of themixture to the mass before normal hydrocarbons of at least the highestcarbon number appear in substantial proportion in the effluent from themass; (3) passing a gaseous eluting material through the zeolite mass toelute from about one-tenth to about one-half of the retainedhydrocarbons from the whole of the zeolitic mass as a normal hydrocarbonenriched stream; and (4) repeating the cycle of steps 1, 2 and 3.

2. A process in accordance with claim 1, wherein the hydrocarbon mixtureis a gasoline boiling range reformate hydrocarbon mixture.

3. A process in accordance with claim 1, wherein the process is operatedat a pressure of from about one to about 1000 p.s.i.g.

4. A continuous process for the separation of normal hydrocarbons from afeed mixture of a plurality of normal hydrocarbons of at least fourcarbon atoms per molecule with non-normal hydrocarbons of similarboiling range which comprises: (1) establishing a continuous flow of aninert gas through (a) a particulate mass of a solid zeolitic materialhaving a rigid three-dimensional anionic network and havingsubstantially uniform intracrystalline interstitial channels of fromabout 5 to about 6 Angstrom units diameter, while maintaining a contacttemperature of from 250 to 600 C., (b) a first hydrocarbon recoverymeans comprising a condenser, and (c) a second hydrocarbon recoverymeans; (2) periodically adding to said gas, upstream from said mass, aportion of said feed mixture to produce a vapor mixture, which passesinto said mass; (3) continuing said addition for a period no longer thanthat required for those normal paraflins desired to be separated toappear in the vapor mixture at the outlet and of said mass insubstantial proportion; (4) recovering hydrocarbons, substantiallyreduced in content of those normal hydrocarbons desired to be separated,in said condenser by maintaining it at a temperature sufiicient tocondense hydrocarbons from the vapor stream during the period When feedis charged to the mass, and (5) recovering mainly normal hydrocarbonsdesired to be separated in said second recovery means, during the periodwhen feed is not being charged to the mass.

5. A continuous process for the separation of normal hydrocarbons from afeed mixture of a plurality of normal hydrocarbons of at least fourcarbon atoms per molecule with non-normal hydrocarbons of similarboiling range which comprises: (1) establishing a continuous flow of aninert gas through (a) a particulate mass of a solid zeolitic materialhaving a rigid three-dimensional anionic network and havingsubstantially uniform intracrystalline interstitial channels of fromabout 5 to about 6 Angstrom units diameter, while maintaining a contacttemperature of from 250 to 600 C., (b) a first condenser maintained at afirst temperature, and (c) a second condenser maintained at a secondtemperature; (2) periodically adding to said gas, lip-stream from saidmass, a portion of said feed mixture to produce a vapor mixture; (3)continuing said addition for a period no longer than that required forthose normal paraffins desired to be separately recovered to appear inthe vapor mixture at the outlet end of said mass in substantialproportion; (4) recovering hydrocarbons, substantially reduced incontent of normal hydrocarbons desired to be separated, in said firstcondenser by maintaining it at a temperature sufficient to condensehydrocarbons from the vapor stream during the period when feed ischarged to the mass, and (5) recovering mainly normal hydrocarbonsdesired to be separated in said second condenser by maintaining it at alower temperature suflicient to condense hydrocarbons from the vaporstream during the period when feed is not being charged to the mass.

6. A cyclic process for the separation of a normal hydrocarbon fromnon-normal hydrocarbons, said normal hydrocarbons having from about fiveto about .ten carbon atoms per molecule, which comprises (1) passing avapor stream of a mixture of said hydrocarbons into a particulate fixedmass of a solid zeolitic material having a rigid three-dimensionalanionic network and having substantially uniform intracrystallineinterstitial channels of from about 5 to about 6 Angstrom units diameterwhile maintaining a contacting temperature of from 250 C. to 600 C. toselectively retard the passage of the normal hydrocarbon through themass while the non-normal hydrocarbons pass through the mass and arerecovered as a non-normal-hydrocarbon-enriched product; (2)discontinuing passage of the mixture to the mass before the normalhydrocarbon appears in substantial proportion in the effluent from themass; (3) passing a gaseous elutiug material through the zeolite mass toelute from about one-tenth to about one-half of the retained hydrocarbonfrom the whole of the zeolitic mass as a normal hydrocarbon enrichedstream; and (4) repeating the cycle of steps 1, 2 and 3.

References Cited in the file of this patent UNITED STATES PATENTS2,306,610 Barrer Dec. 29, 1942 2,522,426 Black Sept. 12, 1950 2,586,889Vesterdal et al Feb. 26, 1952 2,628,933 Eagle et al. Feb. 17, 19532,644,018 Harper June 30, 1953 2,651,603 Martin et al. Sept. 8, 19532,818,137 Richmond et al. Dec. 31, 1957 2,818,455 Ballard et al. Dec.'31, 1957 2,859,173 Hess et al. Nov. 4, 1958 2,859,256 Hess et al. Nov.4, 1958 OTHER REFERENCES Article by Barrer, Quarterly Reviews of theChemical Society (London), vol. 111, 1949, page 300.

Chemical Engineering News, Nov. 29, 1954 (vol. 32), p. 4786, (article,Selective Adsorption With Zeolites).

1. A PROCESS FOR THE SEPARATION OF NORMAL HYDROCARBONS FROM A MIXTURE OFA PLURALITY OF NORMAL HYDROCARBONS OF AT LEAST FOUR CARBON ATOMS PERMOLECULE WITH NON-NORMAL HYDROCARBONS OF SIMILAR BOILING RANGE WHICHCOMPRISES: (1) PASSING A VAPOR STREAM OF THE HYDROCARBON MIXTURE INTO APARTICULTE FIXED MASS OF A SOLID ZEOLITIC MATERIAL HAVING A RIGIDTHREE-DIMENSIONAL ANIONIC NETWORK AND HAVING SUBSTANTIALLY UNIFORMINTRACRYSTALLINE INTERSTITIAL CHANNELS OF FROM ABOUT 5 TO ABOUT 6ANGSTROM UNITS DIAMETER WHILE MAINTAINING A CONTACTING TEMPERATURE OFFROM 250*C. TO 600*C. TO SELECTIVELY RETARD THE PASSAGE OF AT LEAST THEHIGHER MOLECULAR WEIGHT NORMAL HYDROCARBONS THROUGH THE MASS WHILE THEREMAINING HYDROCARBONS PASS THROUGH THE MASS AND ARE RECOVERED AS ANON-NORMALHYCARBON-ENRICHED PRODUCT, (2) DISCONTINUING PASSAGE OF THEMIXTURE TO THE MASS BEFORE NORMAL HYDROCARBONS OF AT LEAST THE HIGHESTCARBON NUMBER APPEAR IN SUBSTANTIAL PROPORTION IN THE EFFUENT FROM THEMASS, (3) PASSING A GASEOUS ELUTING MATERIAL THROUGH THE ZEOLITE MASS TOELUTE FROM ABOUT ONE-TENTH TO ABOUT ONE-HALF OF THE RETAINEDHYDROCARBONS FROM THE WHOLE OF THE ZEOLITIC MASS AS A NORMAL HYDROCARBONENRICHED STREAM, AND (4) REPEATING THE CYCLE OF STEPS 1, 2 AND
 3. 6. ACYCLE PROCESS FOR THE SEPARATION OF A NORMAL HYFROCARBON FROM NON-NORMALHYDROCARBONS, SAID NORMAL HYDROCARBONS HAVING FROM ABOUT FIVE TO ABOUTTEN CARBON ATOMS PER MOLECULE, WHICH COMPRISES (1) PASSING A VAPORSTREAM OF A MIXTURE OF SAID HYDROCARBONS INTO A PARTICULATE FIXED MASSOF A SOLID ZEOLITIC MATERIAL HAVING A RIGID THREE-DIMENSIONAL ANIONICNETWORK AND HAVING SUBSTANTIALLY UNIFORM INTRACRYSTALLINE INTERSTITIALCHANNELS OF FROM ABOUT 5 TO ABOUT 6 ANGSTROM UNITS DIAMETER WHILEMAINTAINING A CONTACTING TEMPERATURE OF FROM 250*C. TO 600*C. TOSELECTIVELY RETARD THE PASSAGE OF THE NORMAL HYDROCARBON THROUGH THEMASS WHILE THE NON-NORMAL HYDROCARBONS PASS THROUGH THE MASS AND ARERECOVERED AS A NON-NORMAL-HYDROCARBON-ENRICHED PRODUCT, (2)DISCONTINUING PASSAGE OF THE MIXTURE TO THE MASS BEFORE THE NORMALHYDROCARBON APPEARS IN SUBSTANTIAL PROPORTION IN THE EFFLUENT FROM THEMASS, (3) PASSING A GASEOUS ELUTING MATERIAL THROUGH THE ZEOLITE MASS TOELUTE FROM ABOUT ONE-TENTH TO ABOUT ONE-HALF OF THE RETAINED HYDROCARBONFROM THE WHOLE OF THE ZEOLITIC MASS AS A NORMAL HYDROCARBON ENRICHEDSTREAM, AND (4) RETAINED HYDROCARBON FROM THE WHOLE OF THE ZEOLITIC MASSAS A NORMAL HYDROCARBON ENRICHED STREAM, AND (4) REPEATING THE CYCLE OFSTEPS 1, 2 AND 3.